1. Field of the Invention
The present invention relates to an improved hold down device for fastening a workpiece to the surface of a machine tool fixture plate. More particularly, the invention relates to a device that converts a straight through bored hole in the workpiece to temporarily act as a counterbored hole for bolting the workpiece to be machined to the work supporting surface with the head of the hold down bolt recessed.
2. Description of the Prior Art
The method used to attach workpieces to be machined to a work support surface when the heads of the hold down bolts had to be recessed was to counterbore each hole, and often both sides of each hole, when the workpiece was to be worked on both sides. For accuracy, the workpiece must be precisely positioned for both counterboring operations. Thus, the set-up and machining time before actual machining of the workpiece was significant. Particularly, when the workpiece was quite bulky and/or heavy, accidents during turn-over of the workpiece may have caused loss of workpieces due to the requirement for multiple handling. The present invention eliminates the counterboring operation for hold down bolts thereby reducing set-up time and increasing productivity.
3. Statement of the Prior Art
The following patent is cited as the most pertinent prior art of which the applicant is aware:
______________________________________ U.S. PAT. NO. NAME DATE ______________________________________ 2,403,330 D. C. Benton 2 July l942 ______________________________________
U.S. Pat. No. 2,403,330 to D. C. Benton on "Blind Fastening Bolt and Nut" relates to a bolt, sleeve and nut combination used to attach two plate members together. In operation, the sleeve and nut are placed upon the bolt. Then the assembled bolt unit is inserted through an inline bore extended through a top and bottom plate. After insertion, the head of the bolt rests on the top surface of the top plate and a taper provided on a sleeve extends through the bottom surface of the bottom plate. The nut is provided with a tapered portion conforming to the taper on the sleeve, and the tapered part of the nut is provided with a plurality of slots dividing the tapered end of the nut into sections which spread as the nut is tightened, thereby engaging the bottom surface of bottom plate and drawing the two plates together in a mating relationship.
The only relevance between the above cited patent and the present invention is that each uses a sleeve having a taper located on one end. However, the taper on the sleeve used by Benton is used to spread the serrations located on the nut thereby allowing the nut to pull two parts together while the taper in the sleeve of the present invention provides a means for locking the sleeve in a first part and at the same time provides a shoulder for a bolt head to work against in pulling a second part into a mating relationship with the first part.